Butterfly Test Failure

Hi.
I've tried to test many external HDDs by HDDScan Butterfly Test and some of them failed to pass this kind of test.The drives are WD Mybook 3Tb and WD Mybook 4Tb.I guessed first the issue is related to HDD capacity but after testing a Mybook 6Tb that passed the test,I found that there might be another reason.I know how Butterfly test works by seeking 1 and n-1,2 and n-2 sector and so where n is the last sector.My questions are:
1-What's the reason or reasons of failure of this test??
2-Which parameters are related to this test?For exapmle : Speed? Spin up time? or ...?
3-Is there any SMART value related to this test that could be measured or compared with other drives that pass the test?
4-Is that a bad sign for health or performance of drive ??
Thanks.